The rapid growth of the wireless mobile communication markets leads to a demand for a variety of multimedia services in the wireless environment. Thus, the recent progress of high capacity of transmission data and high speed of data transmission to provide the multimedia services promotes research on a multiple antenna system (e.g., a Multiple Input Multiple Output (MIMO) system) enabling the efficient use of a limited frequency.
The multiple antenna system transmits data using channels independent from each other by antenna, thus being capable of increasing transmission reliability and transmission rate compared to a single antenna system with no additional frequency or transmit power allocation. The multiple antenna system allows multiple users to simultaneously share a space resource secured through a multiple antenna, thus being capable of more increasing frequency efficiency.
The multiuser multiple antenna system uses a beamforming technique to reduce interference between users. For example, the multiple antenna system can use a coordinated beamforming technique to reduce interference between users. In order to use the coordinated beamforming technique, a transmit end communicates with multiple users through a linear operation. At this time, the transmit end does not use a unitary matrix as a precoder.
When using the coordinated beamforming technique, a transmit end generates a precoder and postcoder using downlink channel information. The precoder and postcoder have vector or matrix values.
The transmit end generates a precoder and postcoder for reducing interference between users at a time a receive end postcodes a signal precoded and transmitted by the transmit end.
The transmit end transmits a dedicated pilot signal orthogonally distributed to each receive end such that each receive end can recognize a postcoder allocated to itself. For example, when transmitting pilot signals to two receive ends, the transmit end allocates pilot tones to the respective receive ends using a subchannel of a tile structure constructed as shown in FIG. 1 below.
FIG. 1 is a diagram illustrating a subchannel structure of a wireless communication system according to the conventional art.
As shown in FIG. 1, a transmit end performs a communication using a subchannel of a tile structure having a size of 4×3.
The transmit end allocates dedicated pilots to respective receive ends such that the pilots are orthogonal to each other. For example, a transmit end allocates a first tone and a twelfth tone as pilots of a first receive end, and allocates a third tone and a tenth tone as pilots of a second receive end. The transmit end uses tones other than the tones allocated for the pilot signals, as tones for data transmission.
A receive end estimates an effective channel by stream using a pilot signal received from a transmit end. Then, the receive end generates a matched filter using the estimated channel and a characteristic of the coordinated beamforming technique. Here, the receive end uses the matched filter as a postcoder that can obtain a maximum signal to interference and noise ratio.
The multiple antenna system can construct streams of the same number as number of antennas of a transmit end irrespective of number of antennas of a receive end because of the characteristic of the coordinated beamforming technique. That is, the transmit end can recognize a plurality of receive ends as respective receive antennas and simultaneously construct streams for the respective receive ends. The transmit end can increase a transmission rate of a system by being able to construct a multiple stream.
However, an increase of number of receive ends serviced by the transmit end results in an increase of dedicated pilots allocated to the respective receive ends, and thus, resources for the dedicated pilots linearly increase. Here, the multiple antenna system has a problem that with an increase of radio resources for pilot transmission, radio resources for data transmission decrease.